Posterolateral fusion is the standard procedure for treating high grade spinal disorders such as spinal stenosis as well as spondylolisthesis. Despite the wide-spread use of posterolateral fusion as a surgical approach for correcting back pain, numerous problems have been associated with its use. Spinal fusion recipients may be at risk for developing Adjacent Segment Disease (ASD), a condition in which the motion segments adjacent to the fused vertebral segments experience higher rates of degeneration deterioration due to an increase in vertebral loading, higher intradiscal pressures, increased range of motion, and increased facet motion.
Dynamic spinal stabilization has recently emerged as an alternative procedure to treat many degenerative spinal disorders. Existing dynamic stabilization devices restore stability to an injured spine while simultaneously allowing a restricted range of motion. These devices are designed to preserve the integrity of adjacent segments by minimizing the transfer of segment motion and facet joint forces between the stabilized spinal segment and the adjacent spinal segments.
Existing dynamic spine stabilization devices incorporate selectively flexible elements such as flexible cords and intervertebral spacers, or flexible spring rods in order to allow a constrained range of motion to the stabilized spinal segment. To date, no existing dynamic spine stabilization device constrains the rotation of the stabilized segments to a center of rotation that is coincident with a physiological center of rotation. Physiologically representative loading of a spinal segment that is stabilized using a dynamic stabilization device is unlikely to occur unless the rotational motion of the spinal segment passes through the spine's natural center of rotation. The imposition of a non-physiological center of rotation location by existing dynamic stabilization devices may result in alterations to the physiological pattern of tissue stresses and may further increase the likelihood of hardware failure. These altered tissue stresses and non-physiological motion patterns may also be induced in adjacent motion segments, increasing the likelihood of long-term complications, such as ASD, associated with existing stabilization procedures.
In addition, at least some of the existing dynamic spine stabilization devices incorporate pedicle screws in their design. However, the treatment of back pain using pedicle-based implants may pose an increased risk of complications in certain patient populations. To address potential risks of the treatment of back pain using pedicle screw-based stabilization devices, interspinous spacer devices may be used to correct spinal stenosis and facet arthrosis when a less invasive surgical procedure is preferred or when pedicle screw use is unsuccessful.
Interspinous spacers are an appropriate treatment for patients experiencing neurogenic pain that is relieved in flexion and exacerbated in extension. Existing interspinous spacer designs aim to unload the intervertebral disc and increase the neuroforaminal height by limiting the amount of motion available during extension. These existing interspinous spacers typically focus closely on the stabilization of extension movements while neglecting lateral bending, axial rotation, and sometimes even flexion movements. As a result, many existing interspinous spacer devices provide limited stability in lateral bending and axial rotation. A variety of attachment methods are used for existing interspinous spacer devices including polyester tethers and metal clamps, and it is unclear how these fasteners may influence the motion segment's center of rotation during spinal flexion movements. In addition, device slippage and spinous process failure are potential complications associated with the use of existing interspinous spacer devices.
There is a need in the art for an interspinous spacer device that not only allows limited motion of injured or deteriorated vertebral segments, but that constrains that motion to a range that is consistent with the range of motion of the corresponding normal healthy vertebrae. In particular, a need exists for an interspinous spacer device in which the degraded vertebrae are constrained to rotate about an axis that is consistent with a normal healthy spine.